1. Field of the Invention
The present invention relates to a driving device which is applied to a linear drive motor.
2. Description of the Related Art
Linear drive motors have been used in various fields of industry such as camera lens motion, mechanism positioning in machine tools, and slide seat motion in automobiles. As a first example of the prior art linear drive motor, there is a shaft moving type motor shown in FIG. 11 (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. H06-078494).
FIG. 11 is a perspective view of the shaft moving type motor according to the first example of prior art.
In FIG. 11, the shaft moving type motor 400 is comprised of a screw member 411 having a thread, and a motor casing 403 accommodating a stator and a rotor that has a rotor shaft formed at its inner periphery with a spiral groove for engagement with the thread on the screw member 411. The screw member 411 is prohibited from rotating by a detent 421 provided at the motor casing 403 and adapted to be engaged with an elongated groove 420 axially formed in the screw member 411. With normal and reverse rotation of the rotor, the screw member 411 can be axially reciprocated.
However, in the shaft moving type motor 400, the screw member 411 and the stator are in mechanical contact with each other, and therefore, there is the problem that wear and noise are easily caused and the screw member 411 cannot be moved at a high speed. In this regard, there is proposed a second example of the prior art motor (see e.g. Japanese Patent No. 3434430).
FIG. 12 is a perspective view showing a configuration of the second example of the prior art motor.
In FIG. 12, the motor 500 is comprised of a motor shaft 502 having a magnet 501 which has an outer peripheral surface thereof provided with a plurality of band-shaped spiral magnetized portions, and a stator 511 having an inner peripheral surface on which spiral ridges are formed after the spiral magnetized portions. The motor 500 generates a rotating magnetic field by sequentially switching directions of current supply to coils (not shown) wound around an outer peripheral surface of the stator 511, thereby causing the magnet 501 to move rectilinearly in an axial direction or rotate around the axis of the stator 511 to follow the spiral ridges formed at a magnetic pole part 512.
With the above described motor 500 where the magnet 501 can be moved out of contact with the stator 511, high speed movement of the magnet 501 can be achieved while causing less wear and noise.
However, the motor 500 requires forming the magnet pole part 512 in the complicated spiral shape on the inner peripheral surface of the stator 511 after the spiral magnetized portions of the magnet 501. This poses the problem that the motor 500 is difficult to manufacture using a machining method suitable for mass production, making it difficult to achieve cost reduction. Besides the axial length of the magnet 501 requires to be equal to the required rectilinear moving amount of the motor shaft at the minimum, and therefore, the magnet 501 becomes long when the required rectilinear moving amount of the motor shaft 502 is large.
When manufacturing the magnet, there are needed a magnetizing yoke having magnetic pole teeth which have nearly the same length as the magnet and a similar shape to that of the magnetized portions of the magnet, and a coil for exciting the magnetizing yoke. Therefore, when the angle which defines the spiral shape of the magnetized portions, namely, the angle θ at which the ridges of the magnetic pole part 512 extend relative to the axis of the stator 511 is made small (see FIG. 12) in order to manufacture the long magnet 501, the magnetizing yoke becomes difficult to manufacture. Especially when the spiral magnetized portions extend for a longer length than the entire circumference of the motor shaft, it becomes difficult to wind the coil on the magnetizing yoke, resulting in ununiform coil winding or the like which poses problems that a variation in permanent magnetization easily occurs, casing a variation in motor torque and increase in cost. Therefore, the above-mentioned angle θ preferably has a large value when manufacturing a long magnet.
In FIG. 12, a force f exerted on the magnet 501 from the stator 511 when the motor 500 is driven is comprised of an axial force component f1 and a perpendicular force component f2. When the magnet 501 is moved, the axial force component f1 provides a propulsive force for the rectilinear movement. Therefore, in order to increase the propulsive force for the rectilinear movement of the magnet 501, a propulsive force for the rotational movement (the perpendicular force component f2) must be small. Consequently, the inclination angle θ of the magnetic pole part 512 relative to the axis of the stator 511, namely, the inclination angle θ of the magnetized portions of the magnet 501 relative to the axis of the magnet 501 preferably has a small value.
With decrease in the inclination angle θ, however, the width of the ridges of the magnetic pole part 512 becomes smaller, and as a result, a sufficient mechanical strength of the motor 500 cannot be secured.
As described in detail above, the inclination angle θ which satisfies the requirements of both the propulsive force for the rectilinear movement of the magnet 501 and the mechanical strength of the motor 500 is difficult to attain in practice, and therefore, it is difficult to manufacture the motor 500 including the long magnet 501. Further, the motor 500 requires the stator coils disposed radially outwardly of the magnet 501, which is disadvantageous for reduction in motor diameter.